High temperature oxidation resistance combined with excellent mechanical and tribological properties are typical requirements for state of the art protective coatings.
The growing demands in forming and machining operations, especially in dry machining, typically require further optimizations of well-established coating systems such as TiN, Ti1-xAlxN, and Cr1-xAlxN. Major failure mechanisms in dry high-speed cutting are flank and crater wear, caused through abrasion, adhesion, as well as tribooxidation, and surface fatigue, which all limit the service lifetime of coated tools. Different studies pointed out that such diverse requirement profiles can be controlled through the application of high temperature self-lubricating coatings such as TiAlN/VN, Si3N4—BN, TiC—C, or MoS2.
Solid lubricants are characterized by phases exhibiting easy shear planes, where Magnéli phase oxides (e.g., V2O5, TiO2, and MoO3) are typical examples for high-temperature applications (500 to 1000° C.). Especially, molybdenum is well-known to easily form various Magnéli phases, MonO3n-1, in a wide temperature range starting at 400° C. However, the problem with Mo is the pesting phenomena (i.e., the formation of volatile oxides) leading to an inferior oxidation resistance due to the lack of dense, adherent, and stable oxide scales. Previous studies in the field of high-temperature bulk materials as well as physical vapor deposited coatings highlight that alloying Mo with Si and B results in excellent oxidation resistance as well as thermal stability, even up to 1300° C. The obtained phases, Mo3Si, T1-Mo5Si3, and T2-Mo5SiB2, do not completely inhibit the formation of MoO3, but clearly reduce its volatility. In the low temperature regime, between 650 and 750° C., a slightly porous borosilica scale is formed protecting the underlying material. With increasing temperatures, the borosilica is depleted in B, and the formation of a denser SiO2 scale is promoted. The formation of volatile MoO3 from Mo—Si—B based materials decreases with increasing temperature. Crucial for this behavior is the appropriate phase combination (MoSi3, Mo5Si3, and Mo5SiB2) and chemical composition within the Mo—Si—B system in general. To combine highest oxidation resistance with excellent mechanical properties, the chemical composition of Mo1-x-ySixBy thin films should fulfill the requirement of y/(x+y)≈0.25 with x+y≧0.35. Thereby, hardnesses of 20 GPa and excellent oxidation resistance (˜500 nm consumed layer thickness even after 1 h oxidation at 1300° C.) can be obtained. Other molybdenum based systems for enhanced tribological properties are architectural arrangements with MoS2, Mo2N, or MoCN.
WO2014037072A1 discloses coatings containing Mo on tools used for direct hot forming. It is proposed to apply on the tool to a coating system, which contain one or more layer packages comprising a high-temperature-active lubricating layer which with increasing distance from the substrate follows a high-temperature-stabilized layer (called also HT-layer in WO2014037072A1).
Specifically, the coating system according to WO2014037072A1 comprises a layer system made of alternating molybdenum-rich and molybdenum-poor layers, wherein the molybdenum-poor layers are the HT-layers having for example a chemical composition given by the formula (MeWO1, MeWO27 MoWOa)N, and the molybdenum-rich layers are the lubricating layers having for example a chemical composition given by the formula (MeWO3, MeWO4,MoWOb)N, with 0≦WOa<WOb<1 and MeWO1, MeWO2, MeWO3 and MeWO4 being elements selected from Al, Cr, Ti, and preferably MeWO1=MeWO3 and/or MeWO2=MeWO4. WO2014037072A1 teaches furthermore that the molybdenum-rich layers can also comprises one or more elements selected from C, O, Si, V, W, Zr, Cu and Ag for improving lubrication, while the molybdenum-poor layers can also comprises one or more elements selected from Si, W, Zr and B for improving high temperature stability.
Previous studies in the field of PVD processed low friction coatings pointed out the possibilities of architectural designs, such as multilayer or nanocomposite coatings (e.g. TiAlN/VN, TiC—C), to combine particular properties and hence gain superior tribological properties.